Conventional wisdom says you shouldn't announce your pregnancy until you are in the 2nd trimester when it's "safe to tell". But, for women with factor v leiden, late losses are a lot more common than they are for the average woman (who's chance of loosing the pregnancy after a heartbeat is heard drops to about 5%). I've lost 2 babies after passing the 1st trimester, well after first hearing the heartbeat, and after it was supposedly "safe" to tell.
Pros of telling early:
-support if you have a loss
-support if your pregnancy progresses, but is difficult
-lets you more openly enjoy the pregnancy early
Pros of waiting:
-fewer people to "untell" if you have an early loss
-less likely you'll hear unsupportive comments after multiple losses
I tend to announce early, but understand when couples feel like the pregnancy is too fragile or tentative to shout from the rooftops.